Radio communication systems often suffer from the effects of multipath propagation also known as multipath fading, whereby a transmitted signal reaches a receiver via a plurality of distinct paths from the transmitter owing to the transmitted signal being scattered and reflected by numerous physical objects. In mobile communication systems, the different paths and their relative signal strengths will change rapidly if the mobile terminal is moving. The receiver receives a superposition of these signals on the different paths, which can add up constructively or destructively, causing variations in the received signal amplitude. If the mobile moves over a distance of the order of half a wavelength (which is 7.5 cm for 1,8 GHz) the interference may change from destructive to constructive or vice versa. A receiver moving at high speed can pass through several fades in a small period. In a bad case, a receiver may stop receiving altogether at a particular location at which the received signal is in a deep fade. Maintaining good communications can then become very difficult. To address this, the use of various diversity techniques (such as antenna diversity, polarisation diversity, time diversity and others) is known to mitigate the negative effect of channel fading. Diversity methods used in current mobile communications system are frequency and time diversity whereas space and polarisation diversity are not used (at least in receivers for mobiles) because of limitations on size, power consumption and processing capacity. In antenna diversity, two or more receiving antennas are provided for a receiver. Provided the antennas are sufficiently separated so that the signals received at one antenna are substantially uncorrelated with those received by another, when one antenna is in a null, another antenna is likely to be able to receive a good signal.
The Rake receiver is one known way of improving reception performance by collecting signal energy of resolvable multipath interference received on one antenna. However, to have a multipath gain the time delay difference between two paths must be larger than a chip's period (about 270 ns, where chip is defined as part of a spreading code used for CDMA coding). This condition is true in some environments (typically outdoor environment), in other environments of type such as indoor or pedestrian (i.e. Base Station outdoor and mobile indoor), the multipath gain is low and the rake performance rapidly degrades. To improve the performance, it is known to use a dual antenna receiver to exploit space diversity.
The number of multipaths resolvable with a rake receiver is:L=└τ/Tc┘+1
where L is the number of multipath paths, τ is the delay spread, Tc is the chip duration. When τ>Tc (indoors or factory) the multipath diversity is null and the performance degrades rapidly. Simulations have been done in outdoor and indoor environment and show the influence on Rake performance. To resolve this problem, diversity is used but to improve the performance it is necessary to have uncorrelated fading on each antenna. The correlation coefficient decreases as the antenna separation increases.
In a UMTS downlink the signals are transmitted in a synchronous manner, so the spreading codes are perfectly orthogonal at the base station. However, orthogonality of spreading sequences is degraded or destroyed by the different delays caused by multipath propagation resulting in multiple access interference MAI at the output coherent rake receiver. This effect is due to the suboptimal treatment of the MAI as an uncorrelated noise by the rake receiver.
In situation where a small number of users are active (Nuser/SF≦0.25), the classical Rake receiver performs in an adequate manner and more signal processing may be unnecessary (assuming high spreading factors 256 or 128). However as the number of users increases to a value approaching the spreading factor, it can have a catastrophic effect on performance even in perfect power control conditions. An interference rejection technique can be used in this case.
PCT patent application WO 0113530 shows an example of applying a Rake receiver to a CDMA system. In this case, a signal received from all L antenna branches is brought via radio frequency parts to a delay estimator connected to the antenna branch. In the delay estimator, the delays of the best audible multipath propagated signal components are searched for. A Rake finger is allocated for processing the found multipath propagated signal components. The delay estimator informs each Rake branch of the delay found. The delay estimator comprises a matched filter for each antenna branch Thus the number of matched filters is also L. In the matched filter a predetermined number of parallel correlation calculations are performed for the received radio signal by different delays in order to estimate the delays of the multipath propagated signal components. In correlation calculation, the spread pilot part contained in the received radio signal is despread by a known spreading code using a predetermined delay.
On the basis of the calculated correlations, an allocator situated in the delay estimator selects at least one delay, by which a multipath propagated signal component is received. The allocator allocates a Rake finger for processing the signal component found by informing the Rake finger of the delay found. To perform the selection, the correlation results of each matched filter are typically combined in the allocator. If the correlation is high, a delay is found that represents the delay of the multipath propagated signal component of the radio signal coming to the antenna branch in question.
This considerably increases the complexity in all situations and so is not practical for a mobile station for which low computational load, low power consumption and low cost are needed.